The present invention generally relates to a semiconductor laser and more particularly, to a semiconductor laser device of a distributed feedback type or distributed Bragg reflection type which may be oscillated in a single longitudinal mode.
In cases where a semiconductor laser device is to be utilized as a signal source for a large capacity and long distance light transmitting system employing optical fibers or as a measuring optical power output device for use in a measuring work, etc., it is desirable that such a semiconductor laser device has operating characteristics for oscillation in a single longitudinal mode. In semiconductor laser device structures of which the longitudinal mode is simplified, the structure which has conventionally been regarded as the most superior is the arrangement in which a diffraction grating having a periodical concave and convex configuration is formed in a light propagating direction at an active region or in the vicinity of the active region, thereby to obtain laser oscillation at a predetermined wavelength to be determined by a pitch of the diffraction grating and a light propagating constant or velocity within an optical waveguide, by causing the above diffraction grating to be coupled with light.
The structure as referred to above may be broadly divided into the distributed Bragg reflection type which utilizes the diffraction grating as a reflector which reflects only light with a specific wavelength, and the distributed feedback type which obtains feedback with respect to light with a specific wavelength.
Subsequently, the structure of a conventional distributed feedback type semiconductor laser device will be described with reference to FIGS. 1 and 2, in which FIG. 1 is a perspective view of a conventional InGaAsP semiconductor laser device, and FIG. 2 is a diagram showing on an enlarged scale, a diffraction grating portion in the semiconductor laser device of FIG. 1.
As shown in FIG. 1, the InGaAsP semiconductor laser device includes an n-type InP substrate 1 having a (001) face formed with a periodical concave and convex configuration at a crystal growth face side thereof, and an optical guide layer 2 of n-type InGaAsP, a n-type InP buffer layer (or cladding layer) 3, a non-doped InGaAsP active layer 4, a p-type InP cladding layer 5, a p-type InGaAsP cap layer 6 and a current confinement insulating layer 7 made of SiO.sub.2, which are successively laminated onto said n-type InP substrate 1, with a p-side electrode 8 and a n-side electrode 9 being applied onto upper and lower sides of the device in FIG. 1 to hold the above laminated layers therebetween. Moreover, light exit faces 10 and 11 which emit a laser beam at cleavage faces (110) and (110) are formed as shown. The diffraction grating is formed in a direction parallel to the light exit faces 10 and 11, and constitutes a wave-like member at a constant pitch between the light exit faces 10 and 11. Faces 12 of the diffraction grating are generally of (111)A faces.
The known distributed feedback type semiconductor laser device having the structure as described above is arranged to achieve the single longitudinal mode oscillation through utilization of the fact that the diffraction grating provided in the waveguide which serves as a light emitting region, selectively reflects only light with a specific wavelength. However, if the p-side electrode 8 is continuously formed extending between the both light exit faces 10 and 11, unnecessary light with a wavelength region not reflected by the diffraction grating is to be optically amplified by the so-called Fabry-Perot resonator function in which the light is subjected to a multiple reciprocating motion by being reflected between the opposite light exit faces 10 and 11 which are set as resonant end faces, and therefore, light having wavelengths other than the wavelength determined by the diffraction grating is to be subjected to oscillation by satisfying the conditions for the laser oscillation. Accordingly, in order to suppress the oscillation by the Fabry-Perot resonator function as described above, it has been a conventional practice to employ a structure in which the electrode is partially removed between the resonant end faces 10 and 11 to form a non-conduction region 13 where no carrier is injected. By the above structure, no current flows in said non-conduction region 13 and the light not reflected by the diffraction grating is attenuated at this portion, thereby making it possible to suppress the oscillation due to the Fabry-Perot resonance mode.
However, the structure in which a light absorbing region for attenuating light is formed as described above has such disadvantages that it lacks working efficiency owing to a difficulty in the discrimination between a light emitting region and a light absorbing region, and that the undesirable oscillation in the Fabry-Perot resonance mode can not be sufficiently suppressed due to incapability for perfectly attenuating light.
Incidentally, the faces 12 and 12' of the diffraction grating are normally constituted by (111)A and (111)A faces. In a zinc blend crystal such as InP or the like, surface energy is larger in the order as in [111]A&gt;[001]&gt;[011], and therefore, if the crystal formed with the diffraction grating is left to stand at a high temperature in the crystal growth process subsequent to the optical guide layer 2 after formation of the diffraction grating, the (001) face gradually appears from the periodical concave and convex configuration formed with the (111)A, and (111) faces, and there are cases where the periodical concave and convex configuration completely disappears in an extreme instance.
As described so far, the conventional distribution feedback laser devices have such problems that Fabry-Perot resonance mode is not perfectly suppressed, and that there is a possibility that the diffraction grating disappears during holding at a high temperature prior to the crystal growth.